Melanocortin 4 receptor (MC4R) mutations can result in genetically derived cause of human obesity or metabolic syndrome. MC4R receptor is a heterotrimeric G-protein-coupled receptor, which transduces signals by activating adenylate cyclase. Expressed in hypothalamic nuclei and other neuronal and non-neuronal tissues, controlling feeding behavior and energy homeostasis, MC4R integrates an agonist (anorexigenic) signal provided by the α-melanocyte stimulating hormone (α-MSH), and an antagonist (orexigenic) signal provided by the agouti-related peptide (AGPR).
As shown in FIG. 1, MC4R is a part of the leptin-melanocortin pathway. Leptin is released from adipose tissue and binds to leptin receptors (LEPR) on AGPR- and pro-opiomelanocortin (POMC)-releasing neurons in the arcuate nucleus (ARC) of the hypothalamus. Leptin binding inhibits AGPR release and stimulates the release of POMC, which undergoes post-translational modification by the prohormone convertase PC1/3 to generate a range of peptides, including α-MSH. AGPR binding to MC4R suppresses MC4R activity, while α-MSH binding stimulates the MC4R. Suppressed receptor activity generates orexigenic signal, whereas stimulated receptor activity generates anorexigenic signal. Signals from MC4R modulate feeding behavior through secondary effector neurons.
Humans affected by a monogenic MC4R-caused disorders, e.g., obesity, are mostly heterozygous carriers of mutant human MC4R (hMC4R) gene with an autosomal dominant inheritance and penetrance and expressivity that varies with age and generational influences. The functional consequences of hMC4R mutations can be schematically divided into the following categories: nonfunctional receptor (e.g. due to missense or frameshift mutations), intracellular retention of the expressed receptor, altered basal activity of the receptor, and altered α-MSH stimulation of the receptor.